This invention relates to a piezoelectric ink jet printer head, and more particularly to a printer head employing a laminated piezoelectric element as a piezoelectric transducer.
In a recent printer, a piezoelectric ink jet has been utilized for a printer head. Such a ink jet employs a known method of `drop-on-demand` mechanism, in which an ink chamber separated by a pair of valves changes the volume of the chamber in accordance with a dimensional displacement of a piezoelectric actuator. More specifically, ink in the ink chamber is jetted from one of the valves when the volume of the chamber is reduced. On the other hand, when the volume is increased, ink is supplied from the other valve to the ink chamber. Multiple jetting devices utilizing the ink jet mechanism are mounted close to each other in the printer head. Desired characters and images can be formed by jetting ink from a selected jetting device.
In the prior art piezoelectric ink jet printer head, one piezoelectric actuator is employed for each jetting device. Thus, the structure of the printer head becomes complicated if a number of the jetting devices are densely arranged to attain a wide-ranging printing with high resolution. In this case, many steps are required to construct the printer head, resulting in high cost. Moreover, since there is a constructional limitation for miniaturizing the actuator, it is difficult to make each jetting device smaller in size. Therefore, only limited resolution can be attained with such a printer head.
To solve the above-mentioned problems, a new type piezoelectric ink jet printer head has been recently proposed. In this ink jet printer head, one piezoelectric actuator is mounted on a plurality of ink chambers. Upon operation, only a part of the piezoelectric actuator which corresponds to a selected jetting device is deformed. Such a printer head is disclosed, for example, in U.S. Pat. No. 4,584,590. FIG. 8 shows a section of a piezoelectric actuator 88 during actuating which is employed in the reference. The piezoelectric actuator 88 is provided with negative electrodes 92a, 92b, and 92c, and positive electrodes 94a and 94b on the surface of a single piezoelectric ceramic plate 90. The ceramic plate 90 is polarized in the direction indicated in FIG. 8. When actuating voltage is applied between the positive electrode 94a and the negative electrode 92a and 92b, an actuating electric field is generated in the piezoelectric ceramic plate 90 in the direction substantially orthogonal to the polarization direction. The positive electrode 94a then moves downward as shown in FIG. 8 in accordance with displacement by the slip effect. Drops of ink are thus jetted from the jetting device (not shown) corresponding to the positive electrode 94a. A piezoelectric ink jet printer head employing the piezoelectric actuator 88 as a piezoelectric transducer can be easily manufactured at a low cost. Additionally, such a printer head can attain high resolution.
However, in the piezoelectric actuator 88, the actuating electrodes are mounted only on the surface of the piezoelectric ceramic plate 90. Thus, the direction of the actuating electric field is not perfectly perpendicular to the polarization direction. Apparent piezoelectric constant d.sub.15 becomes small. Therefore, high voltage of about 200 V is required to obtain the necessary displacement. In order to make the direction of the actuating electric field more orthogonal to the polarization direction, a thinner piezoelectric ceramic plate may be employed. However, if the ceramic plate is thin, the strength of the plate is reduced. Moreover, a short circuit by an electric discharge may occur, since the opposite electrodes are arranged at such a small interval, typically about 0.5 mm. If various methods are taken to prevent the short circuit, the printer head becomes heavy and large in size, and the manufacturing cost is increased.